1. Field of the Invention
The present invention relates to a method of and an apparatus for correcting the offset level of a linear image sensor such as a CCD (charge-coupled device) in an image reading system.
2. Description of the Related Art
Image reading systems operate by applying illuminating light to a subject placed on a subject table, guiding light representing image information carried by the subject as reflected or transmitted light to a focusing optical system, and photoelectrically reading the light with a linear image sensor such as a CCD or the like. The subject is read in a main scanning direction by the linear image sensor while at the same time the subject is moved relatively to the linear image sensor in an auxiliary scanning direction that is substantially perpendicular to the main scanning direction, so that two-dimensional image information can be produced.
As schematically shown in FIG. 9 of the accompanying drawings, a general linear image sensor 1 basically comprises a photodetector 2 comprising a linear array of photoelectric transducer elements (hereinafter referred to as "pixels") P and a pixel transfer unit 3 comprising an odd-numbered pixel transfer array 3o and an even-numbered pixel transfer array 3e which are positioned one on each side of the photodetector 2. The pixels P include odd-numbered pixels Po and even-numbered pixels Pe.
The pixel transfer unit 3 is covered with a metal film (not shown) such as an evaporated aluminum film or the like which shields the pixel transfer unit 3 from light L.
The light L which is detected by the photodetector 2 is converted into electric charges by the respective pixels P. The electric charges are successively shifted from the odd- and even-numbered pixels Po, Pe to the corresponding odd- and even-numbered pixel transfer arrays 3o, 3e in response to shift pulses that are periodically generated at constant time intervals. Thereafter, the electric charges are outputted as odd- and even-numbered pixel signals So, Se from odd- and even-numbered output sections 4o, 4e of FDAs (floating diffusion amplifiers) or the like through respective output terminals of the linear image sensor 1.
The odd- and even-numbered pixel signals So, Se are alternately read through an amplifier and a selector switch, and then converted by an A/D (analog-to-digital) converter (not shown) to a one-line digital image signal corresponding to the photodetector 2. The one-line digital image signal is then stored in a line memory or the like.
FIG. 10 of the accompanying drawings shows an application in which the linear image sensor 1 reads a subject 172 in a scanning range as shown while moving relatively to the subject 172 in an auxiliary scanning direction indicated by the arrow Y. The subject 172 has a peripheral solid area 170 having a high-density color such as black and a central blank area 171. An image 175 (see FIG. 11B of the accompanying drawings) of the subject 172 represented by an image signal produced by the linear image sensor 1 includes a front side area 176 and a rear side area 178, which are positioned forwardly and rearwardly, respectively, of the blank area 171 with respect to the auxiliary scanning direction. The front and rear side areas 176, 178, in which each line in the main scanning direction is contained in the peripheral solid area 170, have their density corresponding to the density of the peripheral solid area 170. However, a central area 177 of the image 175, which contains the blank area 171 in the main scanning direction, has its density reduced in regions corresponding to the peripheral solid area 170.
As a result of an analysis of the above problem, the inventor has found that when the linear image sensor 1 reads the central area 177 containing the blank area 171 in the main scanning direction one line by one line, the light L slightly enters the pixel transfer unit 3 covered with the metal film, producing electric charges in the pixel transfer unit 3, and a noise signal due to the produced electric charges is added as an offset to the pixel signal which has been generated by the photodetector 2.